Rewrite createOverlay to use an Overlay View that is stackable

This refactors overlays so that you can have more than one
open at a time. When closed they are removed from the stack
until you escape back to the page content.

PhotoUploaderPreview now creates an instance of Overlay
- this is probably not perfect but it keeps this patchset as
simplistic as possible.

Change-Id: I0a5ff8fa2c2b7c3c68276bead268b25a0c54a7c8
TODO: Make overlays close with hardware back button press
Merge "Rewrite createOverlay to use an Overlay View that is stackable"